This invention concerns thermoforming, a widely practiced manufacturing process involving the forming of preheated thin plastic sheets into molded parts. In this process, the plastic sheets are preheated in an oven, and then positioned aligned with tooling in a thermoforming press, the sheet drawn against tooling mold surfaces, using a pressure differential or vacuum to force the heated plastic sheet into conformity with the mold surfaces. A plug assist opposite the mold tooling is often used to preform the sheet into closer conformity to the mold surfaces when press platens are moved together to assist in the process of thermoforming the sheet against the tooling mold surfaces.
Often, in automated machines, sheet material is fed off a large roll, cut to a proper length, then clamped about its perimeter in a clamping frame. The clamping frame and sheet is advanced into an oven where the sheet is preheated to a forming temperature. The clamping frame is then advanced into a press having platens mounting mold and plug assist tooling which are brought together to cause the heated sheet to be formed into a part.
Vacuum or a differential air pressure is applied at the same time to form the sheet into conformity with the molding surfaces.
After forming, the press is opened and the completed part is removed or transferred to another station for further processing.
In twin sheet thermoforming, two sheets are preheated and formed as described, and then fused together to form a unitary part.
In another version of the thermoforming process, a preheated sheet is drawn onto a preformed porous substrate by a vacuum applied to the undersurface of the substrate. Such process has been used for applying a cover skin layer to an automotive interior trim piece.
The cover layer can be adhesively bonded to the substrate. While heat activated adhesives can provide a superior bond, to have a separate oven and coordinated handling of the substrate as well as the sheets would add considerably to the complexity and cost of the machine.
In automating this process, it is usual to feed sheet material off a roll fed into a loading area where a proper length is cut off, the cut sheet clamped into a clamping frame for transport to an oven and thereafter to the forming press. Sometimes material rolls are wound such that the edge of the web wanders laterally to some extent as the sheet is pulled off the roll. This will cause some mispositioning of the cut sheets which can cause faulty parts to be produced.
It is the object of the present invention to provide the capability of preheating a substrate to bond a thermoformed sheet thereto without requiring separate additional handling of the substrate.
It is a further object of the present invention to provide an accurately controlled feeding of sheet material into a thermoforming apparatus when being unwound from a roll of sheet material.